High voltage distribution cable insulations comprised of cross-linked ethylene polymer (XLPE) compositions were initially expected to last 20-25 years. However, the actual failure rate of these cables has been found to be significantly greater than the anticipated failure rate. Despite the fact that ethylene polymer is considered to be the most moisture resistant polymer available, water has been identified as a contributing factor to this unexpected failure in performance. Tree like patterns, named "water trees" occur on aging when the polymer insulation is exposed to water and electrical stress. Water trees, once initiated, develop slowly and lead to a reduction in the dielectric strength of the insulation and result in cable failures. Accordingly, it is widely accepted that water trees are responsible for the trend of increasing power distribution cable failure rates.
Dicumyl peroxide is by far the most commonly used chemical agent to cross-link polyethylene. The power cable with cross-linkable conductor shield, insulation and insulation shield, is extruded either in a tandem or true triple extrusion process and subsequently cross-linked in a continuous vulcanisation line at high pressures of steam or dry nitrogen at elevated temperatures. This process requires a large capital investment. A more economic alternative is the silane moisture cure process wherein an organo silane grafted PE or a copolymer of an organo silane and ethylene is first extruded with a tin catalyst master batch. Cross-linking is then affected in the presence of moisture at moderate temperatures and pressures. Since the ingress of moisture necessary for cross-linking is essentially diffusion limited, the resultant water content and size of the micro-voids are comparable to those observed in dry cured XLPE cable. Although technical reports on field-aged cables insulated with moisture cross-linked insulation indicate the superiority of this insulation over peroxide cross-linked polyethylene, these are not certainly devoid of water trees. It would therefore be desirable to have a moisture cross-linkable insulation with water tree resistant properties in addition to its special mechanical and electrical attributes.
Several methods to improve the performance of peroxide cross-linked polyethylene insulation against dielectric deterioration by water tree generation and growth have been described in the literature. Several patents relate to the use of an organo-silane, siloxane and silicone additives to inhibit water tree growth in XLPE. European patent (EP 0,099,640) describes a blend of ethylene and vinyl acetate copolymer as a water tree retardant material. This latter reference had been extended to cover ethylene silane copolymers (JP 3-22309). EP 0,410,431 discloses that an ethylene alpha-olefin copolymer with a density not greater than 920 kg/m.sup.3, grafted with a hydrolysable vinyl silane compound could behave as a water tree resistant insulation. Water curable azidosulfonyl modified ethylene polymers are claimed to be water tree resistant in EP 0,150,773. A copolymer of ethylene and methacryloyloxypropyl trimethoxysilane has so far been the only reactor copolymer claimed to be water tree retardant when used as an electrical insulation (JP 82,208,006). Blends of ethylene olefin copolymers and graft polyolefin-siloxanes form the subject of patents JP 86,235,064 and 85,157,105.
However, there remains a need for a moisture cross-linkable insulation composition which when cross-linked has improved resistance to water tree growth.